Production of films



Eek. H, 193 c. w. BONNIKSEN I PRODUCTION OF FILMS Filed Dec. ll, 1933ALGINIC ACIQ WATER 11- NG-L 00 ALewATE 'SOLUTIO N HOPPER %1 8 WATER 13 0DRYING CHAMBER TANNlNG Patented Feb. 11, 1936 UNITED STATES PRODUCTIONOF FILMS Cyril Wilfred Bonniksen, Leamington Spa, England ApplicationDecember 11, 1933, Serial No. 701,924

In Great Britain December 15, 1932 Claims.

This invention relates to a process for preparing flexible films and thelike.

It is an object of this invention to provide an' inexpensive process ofmanufacture of flexible 5 films and the like which leads to theproduction of films similar to the well-known gelatine and cellulosicfilms commonly used for wrapping and decorative purposes and offering analternative also to spun cellulosic filaments or threads as a 1 silksubstitute.

I am aware that the manufacture of transparent soluble and. insolublefilms has been suggested starting from alginic acid or its salts.

Alginic acid is a chemical substance which has some points of similaritywith starch. It is insoluble in water, though prolonged steeping inwater causes it to swell in a way which in some respects recallsgelatine. The substance is an acid and forms salts with the metals (andammonia). Some of these salts are soluble and some insoluble in water,some are coloured and some colourless. The ammonium and magnesium andall the alkali salts are soluble in water to viscous solutions; thesesolutions are transparent and clear. Almost all the remaining salts areinsoluble in water,

Although it is fairly easy to produce alginate threads or films whichare smooth and. up to a point transparent yet the production of suchfilms with a degree of transparency comparable with that commonlyattained in the manufacture of viscose or cellulose ester films orthreads is attended with many difficulties.

One difliculty in spinning or producing transparent and. clear filmsfrom alginic acid or its salts lies in the fact that these substancesand especially their solutions offer a ready home for the growth ofbacteria and degenerate so quickly that the spinning solution and thefilms produced therefrom either rapidly lose alginic acid or becomediscoloured by the bacteria and their byproducts or rendered opaque,irregular and. useless.

Another difiiculty lies in thelfact that alginate acid or its compoundscan exist in more than one form, the forms difi'ering either chemicallyor colloidally and leading to the production of films, threads orfilaments of different physical characteristics. Without expressing anyfinal View on the exact reason for this difference it is convenient toregard it on the theory that alginic acid can undergo a transformationsimilar to what is known as a lactone transformation and the tworesulting classes will herein be distinguished for convenience as the Aform and the B form.

Broadly speaking it may be said that the A form, when spun into sheets,leads to opalescent or non-transparent products while the B form leadsto clear and brilliant products if other conditions are suitable.

Based on these facts the present invention relates to the employment ofcertain precautions in the production of alginate films with the objectof securing highly transparent, that is to say brilliant products.

These precautions are calculated to prevent bacterial action and also toprevent any change from the fB form (which the solutions initially have)to the A form. These precautions may be broadly stated as follows:

(a) Limiting the concentration of alginate in solution. Preferablynot-more than 10% of alginic acid on the weight of the solution isemployed, though larger amounts may be used if the other precautions areadequate.

(b) Forming into films or filaments as rapidly as possible afterpreparation of the solutions;

(0) Evaporating rapidly (for example to dryness in fifteen minutes);

(d) Keeping the solution acid or neutral, or at least only very weaklyalkaline (for example of a pH value between 5 and 6) (6) Keeping theapparatus and solutions sterile for example by a sterilizing agent:

(1) Using ammonia along with alkali hydroxides or carbonates as thesolvent.

One feature of the present invention comprises the manufacture of highlytransparent alginate films consisting in producing an alkaline solutionof an alginate, eliminating bacterial contamination, forming thesolution into films and drying.

A further feature of the invention comprises the manufacture oftransparent alginate films consisting in producingan alkaline solutionof an alginate in the B form hereinbefore referred to, forming suchsolution into films or filaments and drying.

One of the factors which requires control and has an effect both uponthe bacterial action and upon the question of whether the product existsin the A form or the B form is the nature and strength of the solution.Solution should be efiected by means of sodium or potassium hydroxidesin combination together with ammonia, the resulting solution havingpreferably a concentration not exceeding 10% of alginate.

Very concentrated and neutral or alkaline solutions tend to revert tothe A form and to produce opalescent films, although if spread intofilms immediately after making and quickly dried transparent films canbe obtained from relatively strong solutions. Moreover, strong solutionsdo not keep. As time goes on films produced from them by evaporationwill become more opalescent even though all the conditions ofpreparation remain otherwise unchanged. The more concentrated thesolution the more rapidly will it change to a form producing opalescentfilms. An alginate solution may therefore produce transparent films ifused immediately after it has been prepared, and twenty-four hours laterit may be useless for the preparation of transparent films. If thestrength of the solutions is kept down to about 10% of alginic material,however, and the solutions are sterilized as hereinafter describedtransparent films may be produced even after keeping for a month. Itwill be observed that the results of using a solution which has beenallowed by time to change or revert to the "A" form are rather similarto results of using a solution which has been allowed by time to developbacterial'action and it is not always easy or possible to distinguishbetween the results of bacterial action and the results of internalchemical or colloidal change in the material itself, but the variousprecautions herein set forth taken singly or together will suifice toovercome the various causes of nontransparency.

From what has been said it will be appreciated that sterilization per semay not always be necessary if the formation into films is carried outwith adequate speed. Where bacterial action has occurred, however, thesolution may be sterilized and/ or clarified by filtration prior toforming into The alkaline solution may be sterilized by the additionthereto of a suitable bactericide (e. g. a quantity of copper sulphatein a proportion such as to yield 1 part of copper ions by weight toyields an insoluble alginate, for example a solution of a calcium salt.

The following is a description by way of example of particular methodsof carrying the invention into effect, but it is to be understood thatthe invention is not limited in its scope to the particular examplesdescribed:-

The accompanying drawing diagrammatically indicates the steps of theprocess. In the drawing l l represents a mixing bath where alginic acid,water and sodium carbonate or equivalent material are brought togetherand viscous alginate solution prepared. This passes to a hopper I2 whichdelivers a thin film through a slot in the bottom of the hopper to atravelling band 13 having a glazed surface, for example of celluloseacetate or polished metal. Steam coils l4 heat the band and warmed airis blown in at l5 over its surface so that the film dries quickly. It.is stripped at l6 under a roller l1 and passes successively through atanning bath I8, a washing bath l9 and a drying chamber 20 to a rollingspool 2| where the product is wound up.

Source of alginic acid: The alginic acid used was extracted from wetlaminaria digitata by treatment with sodium carbonate solution. Afterdilution and filtration the solution was bleached and sterilized byaddition of sodium hypo'chlorite. Alginic acid jelly was thenprecipitated by adding dilute hydrochloric acid until the solution wasone-sixtieth normal. The jelly was filtered off, washed with water andstored.

Example I 2700 grams of alginic acid jelly containing 12.7% alginic acidand ten days old, was stirred with 2500 cubic centimetres of water, 50grams NazCOs and 130 cubic centimetres ammonia (tennormal). Theresulting solution was spread and evaporated as before.

The products of these examples were clearand brilliant films showing notrace of opalescence' and comparable in transparency with the filmsobtainable from the spinning of viscose or cellulose acetate.

Further examples Solutions of alginic acid (containing 10% alginic acid)were made up with sodium hydroxide and ammonia in varying proportions;but such that the sum of sodium hydroxide and ammonia used in each casewas chemically equivalent to the alginic acid content of the solution.

Proportions or sodium hydroxide and ammonia Solution No.

Sodium hydroxide Ammo Percent Percent I 15 II 30 70 III 60 40 IV 10 Allthese solutions gave clear films when spread within two hours of making.After 20 hours Solutions II and III still gave clear films but SolutionIV gave an opalescent one and Solution I a clear film only whereevaporation was very rapid. After 45 hours the opalescence produced infilms made from Solutions I and IV was more pronounced. The films werecast on glass dried in about fifteen minutes and then stripped off.

To render these films insoluble they may be treated with calciumchloride solution so as to produce calcium alginate. It is important toappreciate that if a transparent film is required it must be in what hasbeen called the B form because in some cases films in the A form appearalmost transparent until they are tanned with the calcium bath, and.then turn opalescent. The film may remain objectionably opalescent whenit has dried though it will be much less so than when wet. A 5% solutionof calcium chloride tans a film on glass at room temperature in aboutten minutes if it is about 0.035 to 0.040 millimetres thick. Strongersolutions tan more slowly but show a less tendency to make films becomeopalescent during tanning.

The following general remarks may prove useful as to the conditions ofevaporation: It is reasonable to suppose that if evaporation is veryslow a stage will be passed through (and maintained for a sufilcienttime) during which the film is concentrated as just described, and thusaos sce 3 a change" to the A form may occur after the film has beenformed, and clear films will not-be obtained. .It is necessary thereforein order to produce clear films by evaporation that the evaporation mustbe rapid "though high temperatures of evaporation tend to produceyellowv or discoloured films. .Thus in some cases dryingv under vacuumor a warm air blast may. be'.

requisite.

As to. the hydrogen ion concentration of the solution it is found thatthis must not be too high. If caustic soda. is used to dissolve thealginic acid a pH value of between and 6 should be observed, and thougha slightly acid solution may yield transparent results if made intofilms quickly enough, say in half an hour, yet after two hours it may beuseless for this purpose. The solution may, however, be made slightlyalkaline with a volatile weak alkali such as ammonia and still be usefulfor the preparation of clear films.

Coming to the constitution of the solvent solutions, sodium carbonate orhydroxide used alone tend to give opalescent films of sodium alginate.This is presumably due to the sodium alginate remaining in the A" formduring evaporation. Sodium hydroxide is preferred to sodium oarbonate inthe preparation of solutions of sodium alginate to be used for producingtransparent films but great care must be taken to avoid any excess ofalkali and the solutions must be used almost immediately. Ammonia ifused alone, will not easily give transparent films, and the type ofnon-transparency produced is different from the opalescence produced byan excess of sodium alkali. A mixture of sodium hydroxide or carbonateand ammonia gives very good results, as the ammonia evaporates quicklyand leaves the film free from alkalinity and the presence of sodiumalginate in sufiioient proportion seems to obviate the secondary effectof the ammonia referred to. The solution keeps better than plain sodiumhydroxide solutions.

Clear films are stronger, especially when wet, than opalescent ones.

From what has been said on bacterial action it will be clear that ifsterilization is only effected to a degree which merely retards thebacterial action but does not entirely prevent it, or if nosterilization is effected or if the solutions used are above a lowdegree of concentration the operations from clear solution to the driedproduct must proceed at an adequate rate in order to ensure that theproduct is not cloudy.

To ensure that no alginate remains in solution for more than this timeentails far more, however, than attention to the time of operation.Absolute cleanliness must be adhered to. All tanks, pipe lines, filtersin the circuit must be kept down to a minimum in size and must becleared out daily. Provision must be made for rapid cleaning as Well asrapid rate of Working, both features peculiar to the use of algin forthis purpose.

Alternatively methods have been discovered whereby alginate solutionsmay be sterilized and maintained sterile; in which case permanentclarity of solutions results.

(1) The initial solution may be sterilized by raising to 100 C. may bepiped into sterile tanks through sterile pipes. The tanks are suppliedwith sterilized air and so on.

(2) The initial solution may have incorporated with it a suitablegermicide. Not all well known germicides are suitable. A preferredstrength of germicide isil part in ten thousand of copper ions in thesolution. Copper sulphate may be the copper salt employed. v

I claim:-

.1. A process for producing from alginic material films freefromopacity, consisting in preparing an aqueoussolution of an alkali-radiclealginate which .solution is substantially both sterile and clear andwhose concentration (expressed in terms of alginiclacid) does not exceedof the weight of the solution, forming the said solution into a filmbefore it has materially changed from the state in which it existed uponfirst formation and then drying the film in not more than fifteenminutes.

2. A process for producing from alginic material films free fromopacity, consisting in preparing an aqueous solution of an alkaliradiclealginate which solution is substantially both sterile and clear andwhose concentration (expressed in terms of alginic acid) does not exceed10% of the weight of the solution, forming the said solution into a filmbefore it has materially changed from the state in which it existed uponfirst formation and then drying the film in not more than fifteenminutes.

3. A process for producing from alginic material self-supporting filmsfree from opacity consisting in preparing an aqueous solution of amixture of alkali-metal and ammonium alginates which solution issubstantially both sterile and clear and wherein the proportions (statedas equivalents) are such that ammonium constitutes not more than 85% ofthe total alkali-radicle present and the overall concentration of themixed alginates (expressed in terms of alginic acid) does not exceed 10%of the weight of the solution, forming the said solution into a filmbefore it has materially changed from the state in which it existed uponfirst formation and then drying the film in not more than fifteenminutes.

4. A process for the production of self -supporting films from alginicmaterial by preparing an aqueous solution of an alginate, forming thesolution into a film and drying the product, characterized bysterilizing the solution so that bacterial action during the process isobviated.

5. A process for the production of films from alginic materialconsisting in preparing an aqueous solution of a mixture of alkali-metaland ammonium alginates, the proportions (stated as equivalents) beingsuch that ammonium constitutes not more than 85% of the totalalkaliradicle present, sterilizing the solution so that bacterial actionduring the process is obviated, forming the sterile solution into a filmand then drying the product.

6. A process for the production of films from alginic material asclaimed in claim 4 wherein the solution is sterilized by the additionthereto of copper sulphate.

'7. A process for the production of films from alginic material asclaimed in claim 5 wherein the solution is sterilized by the additionthereto of copper sulphate.

8. A process for the production of films from alginic materialconsisting in preparing an aqueous solution of a mixture of alkali-metaland ammonium alginates the proportions (stated as equivalents) beingsuch that ammonium constitutes not more than 85% of the totalalkaliradicle present and the overall concentration of the mixedalginates (expressed in terms of a1- ginic acid) not exceeding 10% ofthe weight of the solution, sterilizing the solution so that bacterialaction during the process is obviated, forming the sterile solution intoa film and finally drying the product.

9. A process for the production of films free from opacity as claimed inclaim 1 wherein the pH value of the solution is maintained during theprocess between pHs and pHs.

10. A process for producing from alginic material continuous lengths ofsolidified material of small thickness free from opacity, consisting inpreparing an aqueous solution of an alkali-radicle alginate whichsolution is substantially both sterile and clear and whose concentration(expressed in terms of alginic acid) does not exceed 10% of the weightof the solution, forming the said solution into a continuous lengthbefore it has materially changed from the state in which it existed uponfirst formation and then drying the length in not more than fifteenminutes.

CYRIL WILFRED BONNIKSEN.

